This invention relates generally to optical fiber amplifiers and, more particularly, to techniques for suppression or reduction of stimulated Brillouin scattering (SBS) in fiber amplifiers. SBS is a well known nonlinear phenomenon that affects various types of optical components, including optical fibers. SBS is often explained in terms of three waves that propagate in a fiber: an incident wave, an acoustic wave and a reflected wave (sometimes referred to as the Stokes wave). When the incident wave reaches a certain power threshold, it excites an acoustic wave, which alters the optical properties of the fiber, including its refractive index. These changes result in scattering of the incident wave and creation of the reflected wave.
As a practical matter, the effect of SBS is to attenuate the incident wave and to limit the maximum power that can be transmitted through a fiber. Because SBS has a fairly narrow linewidth, typically less than 50 MHz (megahertz), a standard SBS suppression technique is to broaden the bandwidth of a laser used to generate the incident wave. This effectively reduces the SBS gain below the SBS threshold and limits the undesired effects of SBS. This approach, however, is incompatible with techniques commonly used for high power scaling of multiple fibers. Such scaling usually requires that the individual fibers have a very narrow bandwidth. Therefore, there is a need for a technique that reduces or suppresses SBS in fiber amplifiers that handle narrowband laser signals.
Of the techniques for SBS suppression that have been proposed, many involve modification of the fiber design and structure itself. While these approaches may be useful in some applications, they are difficult to incorporate into fiber configurations in high power amplifiers. Other approaches have utilized external modifications to fibers to achieve SBS suppression. In particular, it is known that temperature variation and mechanical strain can be used to effectively broaden the SBS linewidth and thereby achieve reduction or suppression of SBS. In particular, the SBS resonance frequency has been observed to vary approximately linearly over modest ranges of temperature and strain. The coefficients of proportionality depend on the specifics of fiber design and composition, but have been observed in the range of 1-2 MHz/° C. for a temperature gradient. See, e.g., J. Hansryd et al., J. Lightwave Technology 19, p. 1691 (2001). Strain has been observed to affect the SBS resonance frequency by approximately 100 kHz/μE, where PE refers to “microstrain” (i.e. a fractional change in length of 10−6). See, e.g., N. Yoshizawa et al., “Proposal for stimulated Brillouin scattering suppression by fibre cabling,” Electron. Lett. 27, 1100-1101 (1991). More specifically, researchers in this field have observed a temperature induced enhancement of SBS resonance by a factor of approximately 2.5 for a temperature gradient of 100° C. (see, e.g., Hansryd et al).
Although the experimental use of temperature gradients for SBS suppression is encouraging, unfortunately these results are based on the use of fiber output powers on the order of a few 100 W. In the application of a high power fiber amplifier, for example, the typical SBS threshold is ˜100 W. See, e.g., A. Liem, et al, Optics Letters 28, 1537 (2003). Therefore, scaling up to much higher SBS thresholds of 2 kW, e.g., would require broadening the effective SBS linewidth by a factor of approximately 20, to approximately (20×50 MHz=)1 GHz. Obviously, for very high signal powers the use of temperature gradients for SBS suppression is limited by practical considerations imposed by the fiber temperature.
Alternatively, the use of strain to broaden SBS linewidth also has practical limitations. Achieving a broadening to approximately 1 GHz would require a strain of about 1% (10,000με). Although significant tensile strain in glass fibers is possible, and the strain limit imposed by fracture is approximately 6%, the routine application of large tensile strain to glass fibers raises significant concerns about fiber degradation and reliability. Constant strains applied compressively have been used in fiber Bragg gratings (FBGs), without any apparent degradation. FBGs, however, typically employ quite short fibers. Prior to the present invention no-one has suggested how to apply a fiber compression gradient to relatively long fibers handling high powers. Accordingly, there is still a significant need for a technique that suppresses or reduces SBS in fibers that handle high powers. The present invention is directed to this end.